Downregulation of Nitric Oxide Synthase Activity in Human Platelets by Nitroglycerin and Authentic Nitric Oxide

BACKGROUND: Besides endothelial cells, platelets possess active nitric oxide synthase (NOS) enzyme, which converts L-arginine to NO and L-citrulline. Nitroglycerin (NTG) inhibits platelet aggregation by increasing intracellular cGMP levels, an effect similar to that of NO. In this study we examined the regulation of platelet NOS activity by NTG and authentic NO. METHODS AND RESULTS: Nitric oxide synthase activity was measured as formation of L-citrulline from L-arginine. As expected, incubation of platelet-rich plasma with NTG resulted in a concentration-dependent (40-400 mumol/L) increase in nitrite levels and an increase in cGMP accumulation. Concurrently, NTG was found to exert an inhibitory effect on platelet NOS activity. Exposure of washed platelets to authentic NO also resulted in approximately 50% decrease in NOS activity. Western analysis showed that NTG and authentic NO had no effect on NOS protein expression in human platelets. CONCLUSIONS: This study indicates that NO, either authentic or derived from NTG, decreases NOS activity without affecting NOS protein expression in human platelets. The downregulation of NOS activity may have a bearing on the phenomenon of nitrate tolerance.